Under attack mode puts the browser challenge in front of the whole zone while it is under an application-layer flood or a scraping wave. Every browser navigation without a valid __waf_pass gets the proof-of-work page («Checking your browser before accessing www.example.com»), on api.* names too, and every other request without a pass gets 403 with {"error":"challenge_required",…} and OpenWAF-Mitigated: challenge, never the HTML page. By default the challenge is one bit harder than the name's usual one, and you can also shorten the pass lifetime.
Paths in exempt_paths (webhooks, payment callbacks), IP and ASN allow rules, WAF exceptions with rule_id 0 and verified crawlers skip it, so search engines keep indexing; a request that only claims to be a crawler is challenged. hosts limits it to some names of the zone. Set until and the control plane switches it off at that time as a new zone version, recorded in the history and the audit log.
Turn it on from the zone overview or Security → Verification (1 h, 6 h, 24 h or until you turn it off), with openwaf-cloud ctl zones under-attack example.com on --for 6h --reason "…", with challenge.under_attack in PATCH /v1/orgs/{org}/zones/{zone}/edge, or with the new MCP tool under_attack. Its challenges appear under the under_attack trigger in the Verification analytics.